The present invention relates to a vertical injection apparatus for a die casting machine wherein an injection sleeve is inserted into an injection bore of a die and hot molten metal poured in the injection sleeve is injected into a cavity within the die. Specifically, the present invention is concerned with a die casting apparatus wherein during casting, a portion into which air intrudes is evacuated prior to vacuum evacuation of the cavity of the die.
Casting methods with die casting machines have been widely popularized in the art as typical methods of manufacturing a large number of precision casting products. The casting of this kind is implemented such that a metal mold cavity is filled with molten metal at high speeds and under high pressures. Accordingly, there is a possibility that gas within the cavity is not sufficiently vented, with the result that such a gas is mixed with molten metal to remain within the product in the form of gas pockets. For this reason, in many applications, prior art casting is not suitable for obtaining are particularly required of high quality free from gas pockets.
To solve such an inconvenience, as disclosed in Japanese Utility Model Laid-open Specification No. 57-13873 and so on, the applicant of this invention has developed and proposed a gas vent device for metal die capable of venting or degassing die cavity during casting to eliminate gas involvement, thus making it possible to produce high quality die casting products.
This gas vent device has a gas vent valve within a gas exhaust path for connecting the die cavity to the outside, whereby hot molten metal is injected into the cavity when this valve is opened and upon eompletion of exhaustion of the gas of small mass within the gas vent valve is closed by an inertial force of hot molten metal of large mass from the cavity entering the gas exhaust path, thus preventing the hot molten metal from flowing out. With this device, venting of gas within the die can be securely and easily performed.
Further, the applicant of this application has also developed a decompression or vacuum type die casting method and apparatus wherein the above-mentioned gas exhause path is connected to a vacuum generating device, thereby positively evacuating gas within the die cavity. This pressure reduction type die casting method is to effect decompression by evacuating gas in excess of the amount of gas flowing from the outside of the die into the cavity through a gap, thus promoting degassing within the mold.
However, in the above-mentioned casting method there occurs gas flow from the outside to the inside via a gap between an injection sleeve and a plunger tip because of the difference between the amount of exhaust gas from the cavity and the amount of gas flowing into the cavity. Therefore, when the cavity is evacuated to vacuum, part of the hot molten metal within the injection sleeve is almost bubbled and randomly sucked up into the cavity before injection, thus producing a thin solidification layer along the internal surface of the cavity. Accordingly, once such a condition is produced, satisfactory injection products cannot be obtained. Further, even when injection is desired to be effected, moisture, mold release or lubricating agent etc. outside the cavity is sucked into the cavity, or hot molten metal within the injection sleeve is sucked into the cavity at a phase where the degree of vacuum is not sufficiently raised. As a result, impurity such as lubricating agent will be mixed into the product, thus degrading the quality of the product, or moisture comes in contact with the injected hot molten metal to turn into gas to thereby create gas pockets, thus failing to exhibit sufficient vacuum effect. In addition, because of the fact that hot molten metal is sucked before a pressure within the cavity reaches a sufficient degree of vacuum, there is a possibility that it involves gas thereinto, or admixing of hot molten metal previously injected and that injected subsequently is not sufficiently carried out, thus degrading the appearance of the product.
Further drawbacks of the above-mentioned conventional casting apparatus will be pointed out. When injection is carried out while the inside of the metal mold cavity being evacuated to vacuum from above, air is sucked thereinto via a contact portion of the injection sleeve. Accordingly, when a solidification layer produced at the top of the plunger tip at the initiation of the injection is crushed down at the time of injection, air admitted via the contact portion of the injection sleeve enters into the hot molten metal, with the result that air enters into the cavity of the die along with the hot molten metal. Such an inconvenience will be explained in an illustrated manner with reference to FIG. 18. Into an injection bore 4 of a die 3 having a cavity 1 and a constricted portion 2, a stationary sleeve 5 is fitted and an injection sleeve 7 into which hot molten metal 6 is supplied for injection is inserted. Into an inner hole of the injection sleeve 7, a plunger tip 8 is slidably fitted. At the time of intiation of the injection, a solidification layer 9 is produced due to the fact that the hot molten metal 6 is solidified on the end surface in contact with the hot molten metal of the plunger tip 8 and the internal surface of the injection sleeve 7 contiguous therewith. The solidification layer 9 is pushed up by the plunger rising for injection, and is compressed and crushed down between a plane 10 in front of the constricted portion 2 and the end surface of the plunger tip 8, with the result that it is left along with a biscuit of a solidified material of the hot molten material 6 within the stationary sleeve 5 without entering into the cavity 11. When taking out the product, the solidification layer 9 integral with the biscuit is separated from the constricted portion 2.
However, in the conventional casting apparatus, when gas within the cavity 1 is evacuated to vacuum, air is sucked from a gap betwen the injection bore 4 of the die 3 and the injection sleeve 7 inserted thereinto. Thus, air intrudes into a gap between the internal surface of the stationary sleeve 7 and the solidification layer 9. Further, the solidification layer 9 is compressed by the plunger tip 8 to come in contact with the plane 10. As a result, when the solidification layer 9 is partially broken in succession as indicated by reference numerl 9A in the figure, the admitted air enters into the hot molten metal via gaps between the broken solidification layer fragements 9A and enters into the cavity 1 via the molten metal and the constricted portion 2. Consequently, the air and the hot molten metal alternately pass throggh the constricted portion 2. As a result, the flow of the hot molten metal is disturbed, thus failing to provide high quality casting products.
In view of this, the applicant of this application has proposed a casting method and an apparatus therefor as described in Japanese Patent Laid-open Specification No. 59-144566 etc. wherein vacuum evacuation is implemented from the sliding surface of the plunger tip simultaneously with decompression within the die cavity. The configuration illustrated as an example of the apparatus is such that a groove is formed in the outer peripheral surface of the plunger tip slidable within the injection sleeve and the groove is connected to a vacuum generating device. In this example, for convenience of machining the groove and an air passage, there is employed a so called double-tip to divide the plunger tip in an exial direction whereby a groove is formed at its boundary portion or in the upper outer periphery of one segmental plunger tip, and an air passage is provided which axially penetrating through the other segmental plunger tip and the plunger rod.
However, the drawback with the above-mentioned conventional injection plunger device of the double-tip type is as follows. In the event that, when part of hot molten metal within the injection sleeve is sucked at the time of vacuum evacuation, an air passage is clogged with the hot molten metal, it is necessary to cut the plunger rod at the clogged portion to remove the clogged material or to exchange the faulty plunger rod with a new one, with the result that not only much time and labor for the repairing work is required, but also its cost is increased.